NET ESCAPE PROBABILITY OF CONTAMINANT FROM A LOCAL DOMAIN TO EXHAUST OUTLET

Abstract

 The amount of contaminant re-circulated in a local domain implies the potential of the ventilation efficiency in a room. The returning probability (hereafter RP) of contaminant generated in a local domain and the net escape probability (NEP) of that will become useful information for understanding the mechanism of contaminant concentration distribution formed in a target room and for controlling Indoor Air Quality. Here we proposed the fundamental definitions of RP and NEP, and then discussed the potential relation with net escape velocity ((hereafter NEV, 2013 Lim et al.) concept. Further, the calculation results of RP, NEP and NEV distributions in simple 2D model room were demonstrated.
 We have already reported the definition and calculation procedure of NEV. NEV is defined for control volume of CFD (C.V.) and is calculated based on the net flux, difference between transport out of and into the C.V. NEP is defined as the probability that is exhausted directly through exhaust outlet and does not re-circulate to the target local domain (here, C.V.) again. If the RP, to target local domain is assumed to be constant, is calculated by the summation of geometric series of returning frequency to the target local domain. The sum of NEP and RP equals ‘1.0’ and the NEP must be (1-α).
 In this study, the NEV and NEP distributions were analyzed in 2D simple model room. The size of indoor model is 10L₀×10L₀ (L₀: size of inlet) with a dimensionless scale. We set up the dimensionless wind velocity U_in 1.0 [-] at inlet. For calculating the NEP and NEV in a C.V., contaminant was assumed to be generated in C.V. and this meant that the same numbers of CFD calculations with the number of C.V. within the target room were required.
 As a result, the supplied jet from the inlet opening flowed into the room along the floor and reached the sidewall opposite the supply inlet. Flow bifurcation was observed in front of the exhaust outlet and a large circulating flow and one-pass flow from the supply opening to the exhaust outlet were formed. A stagnant region was formed at the center of the room. The slight differences between air velocity vector and NEV that were calculated by net flux removed/diluted contaminant from the target C.V. was confirmed.
 NEP became ‘1.0’ at the vicinity of exhaust outlet because the contaminant in the target C.V. was directly exhausted through exhaust outlet and would not be returned. It means that RP denotes ‘0’. NEP at the vicinity of inlet was ‘0.94’ because most of contaminant in the target C.V. was transported from inlet to exhaust outlet with little re-circulation. NEP became high at the center, stagnant region, in the room. In this region, the contaminant concentration was high and the driving force of contaminant transportation depended on diffusion than convection. And then diffused contaminant from stagnant region in room did not return to the original C.V.